(a) Field of the Invention
The present invention relates to a display panel, a light emitting display using the display panel, and a driving method thereof. More specifically, the present invention relates to an organic electroluminescent (EL) display panel, a light emitting display using the EL display panel, and a driving method thereof.
(b) Description of the Related Art
In general, an organic EL display panel is a display device for electrically exciting fluorescent and organic compounds and emitting light. In such an organic EL display panel, (M×N) organic emission cells are voltage or current driven to represent images. An organic emission cell includes an anode (typically formed using indium tin oxide (ITO)), an organic thin film, and a metallic cathode layer. The organic thin film includes an emission layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) for balancing electrons and holes to improve emission efficacy. The organic thin film also includes an electron injection layer (EIL) and a hole injection layer (HIL).
Methods for driving the organic emission cells include a passive matrix method, and an active matrix method using thin film transistors (TFTs). The passive matrix method uses anodes and cathodes that cross each other. In the passive matrix method, a line is selected to drive the organic emission cells. The active matrix method uses TFTs that access respective ITO pixel electrodes. In the active matrix method, a line is driven according to a voltage maintained by the capacitance of a capacitor coupled to a gate of a TFT. The active matrix method is categorized, depending on formats of signals applied to the capacitor for establishing the voltage, as a voltage programming method or a current programming method.
FIG. 1 shows an equivalent circuit diagram for a pixel circuit that implements the conventional voltage programming method. As shown in the equivalent circuit diagram of FIG. 1, a transistor M1 is coupled to an organic EL element (OLED) to supply the current for emission, and the current of the transistor M1 is controlled by a data voltage applied through a switching transistor M2. A capacitor C1 for maintaining the applied voltage for a predetermined time is coupled between a source and a gate of the transistor M1.
When the switching transistor M2 is turned on, the data voltage is applied to the gate of the transistor M1 to charge the capacitor C1 with the voltage VGS between the gate and the source, a current IOLED flows though the transistor M1 in response to the voltage VGS, and the OLED emits light in response to the current IOLED.
The current flowing through the OLED is given as Equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                                                                V                      TH                                                                                          )                            2                                                          Equation        ⁢                                  ⁢        1            
where IOLED is a current flowing through the OLED, VGS is a voltage between the gate and the source of the transistor M1, VTH is a threshold voltage of the transistor M1, VDATA is a data voltage, and β is a constant.
As given in Equation 1, the current corresponding to the data voltage is supplied to the OLED, and the OLED emits light in response to the supplied current. The applied data voltage has multiple-stage values within a predetermined range so as to represent gray scales.
The pixel circuit for implementing the conventional voltage programming method has difficulties in obtaining high gray scales because of variations in the threshold voltage VTH and the carrier mobility. Such variations are caused by non-uniformity of a manufacturing process. For example, in order to represent 8-bit (i.e., 256) gray scales by driving TFTs using the voltage of 3 volts (3V), the voltage applied to the gate of the TFT should have an interval of less than the voltage of approximately 12 mV(=3V/256). Hence, if the variation in the threshold voltage of the TFT caused by the non-uniformity of the manufacturing process is 100 mV, it is difficult to represent high gray scales. Also, representing high gray scales is further complicated since the value of β in Equation 1 is not constant because of the variation of electron mobility.
The pixel circuit of the current programming method achieves substantially uniform display characteristics when the driving transistor in each pixel has substantially nonuniform voltage-current characteristics, provided that a current source for supplying the current to the pixel circuit is substantially uniform throughout the whole panel.
FIG. 2 shows an equivalent circuit of a pixel circuit for implementing a conventional current programming method. As shown, the transistor M3 is coupled to an OLED to supply the current for emission, and the current of the transistor M3 is controlled by a data current applied through a transistor M4.
Accordingly, when transistors M4 and M5 are turned on, the voltage corresponding to the data current IDATA is stored in a capacitor C2 coupled between the source and the gate of the transistor M3, and a current corresponding to the voltage stored in the capacitor C2 flows to and through the OLED to emit light. The current flowing through the OLED is given as Equation 2.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                      I            DATA                                              Equation        ⁢                                  ⁢        2            
where VGS is a voltage between the gate and the source of the transistor M3, VTH is a threshold voltage of the transistor M3, and β is a constant.
As given, since the current IOLED flowing through the OLED is proportional to the data current IDATA in the equivalent circuit of FIG. 2, substantially uniform characteristics are obtained provided that the programming current source is substantially uniform throughout the whole panel. However, the current IOLED flowing through the OLED has a small magnitude, and requires a relatively long time to charge a data line with the current IDATA, which also has a small magnitude. For example, several milliseconds are typically required to charge the load of the data line with the data current of about several tens to several hundreds of nano amps (nA), assuming that the capacitance of the data line is 30 pF. As the line time is only several tens of μs, the charging time is too long.
Also, when the current IOLED flowing though the OLED is increased so as to reduce the time used for charging the data line, the total brightness of pixels increases and image characteristics worsen.